Conventionally, in the case where quaternary or higher level light intensity modulation is performed, for example, with regard to a general Mach-Zehnder interferometer (MZI) light intensity modulator 100 as illustrated in FIG. 10, there is a method such that a multilevel intensity-modulated optical signal I(t) is output from an optical waveguide 101 according to a voltage signal V(t), by value multiplexing a level of the voltage signal V(t) applied to an electrode 102 to a quaternary or higher level and driving the electrode 102.
Moreover, in a multilevel light intensity modulation circuit described in Japanese Laid-open Patent Publication No. 2003-258733, there is proposed a technique for realizing multilevel intensity modulation by providing a modulator respectively on a pair of branched waveguides of an MZI optical waveguide, and setting a branching ratio in a branching section of the MZI optical waveguide to 1:2, to thereby change the intensity of an optical signal output from a multiplexing section approximately at a ratio of 0:1:2:3.
However, in the case of the conventional configuration illustrated in FIG. 10, it is generally difficult to value multiplex the level of the voltage signal V(t) provided at a binary level corresponding to “0” and “1” of data to a quaternary or higher level. Moreover, in the case of the MZI light intensity modulator, as illustrated on the left side of FIG. 11, a relation of output light intensity I with respect to applied voltage V (hereinafter, referred to as “electro-optic property”) has a nonlinear characteristic, and the voltage signal V(t) applied to the electrode 102 needs to be multiplexed in an unequally-spaced manner in order to realize equally-spaced light intensity modulation. However, this is even more difficult to realize. Furthermore in the case where a DC bias for adjusting an operating point of the MZI light intensity modulator is shifted from an optimum point, for example, as illustrated in FIG. 12, each level of the light intensity largely changes. Therefore there is also the drawback in that this is not practical. In addition, in the case of the MZI light intensity modulator using lithium niobate (LiNbO3) as a substrate material, drift of the operating point occurs due to temperature change or the like. As a measure against this, the DC bias needs to be controlled. However there is also a problem in that it is difficult to apply known control techniques corresponding to binary light intensity modulation, to quaternary or higher level light intensity modulation.
Furthermore in the case of the conventional technique described in Patent Document 1 above, the modulation becomes multilevel light intensity modulation including zero level (quenching state). Therefore, for example, if application to a multilevel modulation method using optical intensity modulation and optical phase modulation in combination is taken into consideration, there is a problem in that phase information cannot be provided when the light intensity is zero level.